Cooperative traction control system using dual slip controllers

ABSTRACT

A cooperative traction control system that integrates throttle control and torque distribution. The system also uses dual slip controllers and methods that involve controlling the distribution of torque between wheels in the front and rear axles of a vehicle and a relatively small or no adjustment of the engine throttle (or, more generally, engine torque output) to reduce wheel slip. The control is cooperative in the sense that two controllers—a front axle torque controller and a rear axle torque controller—work together (or are controlled together) to reduce wheel slip and thereby achieve improved straight-line movement of a vehicle from a standstill.

RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 61/221,707 of the same title, filed on Jun. 30,2009, the entire contents of which is herein incorporated by reference.

BACKGROUND

Modern four-wheel or all-wheel drive vehicles have been developed toprovide greater vehicle traction over varied terrain and road surfaces.Roads may be dry, wet, icy, snow-covered, or some combination of theseconditions and four-wheel or all-wheel drive vehicles offer advantagesover vehicles in which just two wheels are driven (for example, eitherthe front wheels or the rear wheels). Often, all-wheel drive vehiclesuse an electronically controlled system to affect the way in which thevehicle responds to certain road conditions. For example, electronicsensing units are used to monitor vehicle conditions such as wheelspeed. Such sensing units provide signals to a control unit, which canalter how torque is distributed to the wheels. For example, in manycurrent traction control systems, if wheel slip is detected, throttlecontrol is implemented such that the torque output of the engine isreduced and, as a consequence, the torque to the driven wheels isreduced.

SUMMARY

Although traction control systems are known, they are not fullysatisfactory. For example, many systems do not manipulate torque todrive wheels on the front and rear axles. In addition, many do notintegrate throttle control and torque distribution.

One embodiment of the invention provides what the inventors refer to as“cooperative” traction control, which involves control of thedistribution of torque between wheels in the front and rear axles of avehicle and a relatively small or no adjustment of the engine throttle(or, more generally, engine torque output) to reduce wheel slip. Thecontrol is cooperative in the sense that two controllers—a front axletorque controller and a rear axle torque controller—work together (orare controlled together) to reduce wheel slip and thereby achieveimproved straight-line movement of a vehicle from a standstill.

One embodiment of the invention provides a traction control module thatincludes first and second comparators. The first comparator receives aleft front wheel slip value and a right front wheel slip value. Thesecond comparator receives a left rear wheel slip value and a right rearwheel slip value. Each comparator outputs the larger of the two wheelslip values received. Each of these values is a “front axle slip value”and a “rear axle slip value,” respectively. The traction control modulealso includes first and second summing nodes, one to process the frontaxle slip value from the first comparator and one to process the rearaxle slip value from the second comparator. The output of eachcomparator is provided to a summing node. The summing node for the frontaxle also receives a target slip value for the front axle. The summingnode for the rear axle receives a target slip value for the rear axle.The outputs of the summing nodes represent slip errors and these valuesare provided, respectively, to front and rear axle controllers. Thefront and rear axle controller generates torque command signals based onthe error signals.

The module also includes a third comparator. The output of the frontaxle controller is inverted and sent to the third comparator. The thirdcomparator determines the lesser of the front axle command signal andactual torque provided to the front axle. The lesser of these values isprovided to a third summing node, which also receives the output of therear axle controller. The difference between these values is provided toa fourth summing node which also receives an engine target torque valuefrom an engine controller. The output of the fourth summing node isprovided to the engine to control its overall torque output. The commandsignal from the rear axle controller is provided to a transfer case (orsimilar controllable, torque-distribution device). The two commandsignals have the overall effect of mildly reducing the torque producedby the engine and distributing more torque to the rear axle (than thefront axle) in a situation where the wheel slip of the front wheels isgreater than the wheel slip of the rear wheels.

In another embodiment, the invention provides a method of providingtraction control in a vehicle having a front axle, a rear axle, and anengine that produces torque. The method includes determining a leftfront wheel slip value and a right front wheel slip value; comparing theleft and right front wheel slip values; and generating a front axle slipvalue that is indicative of the greater of the two. A left rear wheelslip value and a right rear wheel slip value are determined and comparedto generate a rear axle slip value that is indicative of the greater ofthe two. Once wheel slip has been evaluated on an axle-by-axle basis, afirst slip error is determined based on the front axle slip value and atarget slip value for the front axle. A second slip error based on therear axle slip value and a target slip value for the rear axle is alsodetermined. A first torque command output is generated by or with afront axle controller based on the first slip error. A second torquecommand output is generated by or with a rear axle controller based onthe second slip error.

The torque commands are used to control the torque of the front and rearaxles. However, the first torque command is modified in a manner thataccounts for 1) the difference in the target torque for the front wheelsand the actual torque and 2) the amount of torque that can be shifted tothe rear wheels, in circumstances where the traction available to therear wheels is greater than the traction available to the front wheels.In one implementation, this adjustment is achieved by comparing thefirst torque command output and an actual front axle torque value andgenerating an excess torque output that is indicative of the lesser ofthe two. A difference output based on the difference between the excesstorque amount and the second torque command is determined. An enginetarget torque value is generated with an engine controller and an enginetorque command is determined based on the difference output and theengine target torque value. The engine torque command is provided to anengine controller to control a torque output of the engine. The commandsignal from the rear axle controller is provided to a controllabletorque distribution device (such as a transfer case) to control theamount of torque provided to the rear axle.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates throttle control used in prior traction controlsystems.

FIG. 1B illustrates drive torque distribution used in prior tractioncontrol systems.

FIG. 2A illustrates throttle control in one embodiment of cooperativetraction control.

FIG. 2B illustrates drive torque distribution in one embodiment ofcooperative traction control.

FIG. 3 illustrates an embodiment of a cooperative traction controlmodule designed for an all-wheel drive vehicle that is primarilyfront-wheel drive.

FIG. 4 illustrates an embodiment of a cooperative traction controlmodule designed for an all-wheel drive vehicle that is primarilywheel-rear drive.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. For example, embodiments describedbelow relate to vehicles in which the front wheels provide the primarymotive force and motive torque is provided to the rear wheels only whencertain conditions exist. However, the techniques described could bereadily applied to other vehicles, including vehicles that are primarilyrear-wheel drive and in which motive torque is provided to the frontwheels under certain circumstances. Thus, in a more general sense,embodiments of the invention are applicable to vehicles with “first” and“second” axles or groups of axles and torque may be controlled based onwhether wheel slip is greater at one of the two axles or groups ofaxles.

FIG. 1A illustrates the operation of a prior vehicle traction controlsystem that relies upon throttle control. Traditionally, throttlecontrol involves mechanically manipulating a throttle that controls theflow of an air-fuel mixture into an internal combustion engine. In manyvehicles, an accelerator or “gas pedal” operated by a vehicle driver isconnected to the throttle to control the amount of air-fuel mixture intothe engine and, as a consequence, the output or torque of the engine. Itshould also be understood that until relatively recently torquedistribution was uniform and non-selective in that drive trains were, atleast in general, designed to provide an equal amount of torque to drivewheels and the ability to control the amount of torque was often limitedto either applying all available engine torque to the driven wheels ornone.

In the context of the current invention, “throttle control” is notstrictly limited to control of a throttle, as modern vehicles mayinclude a variety of mechanisms that control the delivery of air andfuel to an engine in addition to or in place of a throttle. Also, modernvehicles may include electric or other motors whose output is notcontrolled by a throttle controlling an air-fuel mixture, but, forexample, the amount of current provided to the motor. Accordingly,throttle control is used more broadly to indicate controlling the outputof a vehicle engine. A vehicle engine may be an internal combustionengine, an electric motor, a hybrid drive train, a hydraulic motor, orother source of torque.

The left-hand side of FIG. 1A includes a graphic representation 10 ofthrottle control where a certain throttle input 14 is provided to thevehicle engine to cause a vehicle to move from a standstill. As shown inFIG. 1B, in a vehicle with four-wheel or all-wheel drive an amount oftorque (represented by upwardly pointing arrows 15-18) is provided toeach of the vehicle wheels. In FIG. 1B, a situation in which the frontwheels experience more slip or slippage as compared to the rear wheelsis shown (as indicated by the two graphical indicators (crescents in thedrawings) behind each front wheel and one graphical indicator (again, acrescent) behind each rear wheel). Such a situation might arise, forexample, when the front wheels of a vehicle are on an icy spot and therear wheels are located on ice-free pavement. In the system shown inFIGS. 1A and 1B, once wheel slip is detected the throttle input from thedriver is overridden by the traction control system (“TCS”). To reducethe slip of the front wheels, the TCS reduces the amount of torqueprovided to the wheels, by reducing the throttle input from the input 14to a throttle input 22. But as is illustrated in FIGS. 1A and 1B, thetorque is reduced is a non-selective and drastic manner. To reduce theslippage of the front wheels, the system employs a relatively largereduction in throttle input from the input 14 to the input 22. Thisresults in a reduction of the total drive torque (as shown by the arrows24-27, which are shorter than the arrows 15-18) and a first accelerationof the vehicle, A1 (represented by arrow 30), and movement of thevehicle from position P1 to P2.

FIGS. 2A and 2B illustrate vehicle traction control in which thereduction of the throttle input is less than that used in the exampleillustrated in FIGS. 1A and 1B. In the example illustrated in FIGS. 2Aand 2B, throttle input is reduced from the input 14 to an input 36,which is greater than the input 22. In addition to this smallerreduction in throttle input, torque is shifted from the front wheels tothe rear wheels as is shown by arrows 37-40. Arrows 39 and 40 are longerthan arrows 37 and 38, indicating that a greater amount of torque hasbeen applied to the rear wheels. This results in increased accelerationof the vehicle, A2 (represented by the arrow 45), and movement of thevehicle from position P1 to position P3. P3 is farther away from P1 thanposition P2. Thus, improved vehicle launch (from a standstill) isachieved with the system illustrated in FIGS. 2A and 2B.

FIG. 3 illustrates a cooperative traction control module 48 and the flowof information between the module 48 and other control modules in avehicle 50. The vehicle includes an engine 51, a transmission 53, andtransfer case 55 (all shown schematically). As noted above, the enginemay be an internal combustion engine, electric motor, or other source oftorque. Also, multiple engines could be used. For example, an electricmotor could be used at each wheel of the vehicle. The transfer case 55is, in general terms, a controllable, torque-distribution device in thesense that it, in response to a command or signal, distributes torquefrom a source (such as an internal combustion engine) to axles connectedto the wheels of the vehicle. In an embodiment with multiple engines ormotors, the need for a torque distribution device is lessened as thedistribution of torque may be accomplished through, for example,individually controlling each engine.

In the embodiment shown, the module 48 is illustrated as if it and someother components in the drawings are separate from and outside of thevehicle 50 (shown schematically). However, in most implementations, themodule 48, the vehicle controller area network (“CAN”) bus (discussedbelow), and other components are all located within the vehicle 50.Sensors 56 that are part of an electronic stability control (“ESC”)system (and thus, actually located within the vehicle 50) collectinformation about the vehicle such as the rotational speed of each ofthe wheels of the vehicle. The wheel speed information from the ESCsystem sensors 56 can be processed using known techniques (as is shownby processing block 57) to generate four wheel slip values: 58, 59, 60,and 61. The value 58 is the wheel slip for the left front wheel. Value59 is the wheel slip for the right front wheel. Values 60 and 61correspond to the wheel slip for the left rear wheel and right rearwheel, respectively.

The two front wheel slip values 58 and 59 are fed to a first comparator63. The comparator 63 determines the larger of the two slip values 58and 59 and outputs a front axle slip value 64, which represents thelargest amount of slip experienced by the front wheels. In a similarmanner, the two rear wheel slip values 60 and 61 are fed to a secondcomparator 65. The comparator 65 determines the larger of the two slipvalues 60 and 61 and outputs a rear axle slip value 68, which representsthe largest amount of slip experienced by the rear wheels.

The output 64 is sent to summing node 69. The summing node 69 receivesanother input 72 that represents a predetermined or target value forallowable slip at the front axle. The target slip at the front axle 72is an empirical value (i.e., a value determined based on observation orexperimentation). The summing node 69 determines the difference of thetwo inputs 64 and 72 and outputs a value 74 indicating the amount offront axle slip error.

The rear axle slip value 68 is sent to summing node 70. Summing node 70receives a second input 71 that represents a predetermined or targetvalue for allowable slip at the rear axle (which like the input 72 is anempirical value). The summing node 70 determines the difference betweenthe two inputs 68 and 71 and outputs a value 75 indicating the amount ofrear axle slip error. The rear axle slip error 75 is sent to a rear axlecontroller 76. The rear axle controller 76 generates a command signal 77that includes a target torque value for the rear axle. (In FIG. 3, thelabel MSoH_CTCS is used to identify the signal 77).

The front axle slip error 74 is provided to a front axle controller 78.The front axle controller 78 uses the front axle slip error 74 todetermine an amount of torque to apply to the wheels connected to thefront axle. Note that a large (in relative terms) value for the frontaxle slip error 74 is indicative of a relatively large amount of wheelslip difference. In response to a front axle slip error having such avalue, the front axle controller generates a command or output 81 toreduce the amount of torque provided to the front wheels.

When there is slippage, the command signal or output 81 of the frontaxle controller 78 is indicative of an excess amount of torque on thefront axle. (In FIG. 3, the label “Excess Torque VA” is used to identifythe output 81). The output 81 is inverted in an inverter 82 and theinverted value is delivered to a third comparator 83. The thirdcomparator 83 also receives an input 84 that represents the actual ormeasured front axle torque of the vehicle 50. (In FIG. 3, the label“Measured Torque VA” is used to identify the input 84). The comparator83 generates an output 86, which is the lesser of the input 84 and theinverted output 81.

The output 86 is provided to a summing node 87. The summing node 87 alsoreceives the output or command signal 77 of the rear axle controller 76.The summing node 87 determines the difference between the command signal77 (target torque) and the output 86 of the comparator 83. The summingnode 87 generates an output 88 which is the difference between theexcessive torque at the front axle and the additional amount of torquethat can be applied to the rear axle (without slippage at the rearaxle).

The output 88 is sent to a fourth summing node 89. The summing node 89receives an engine target torque value 91 which is a signal generated bya TCS controller 93. The TCS controller 93 generates the engine targettorque value based on upon information from the ESC sensors 56. Thesumming node 89 generates an output 100 that is delivered to CAN bus 102and addressed to an engine controller 105. The command signal 77 is alsorouted to the CAN bus 102 and addressed to the transfer case 55. Thecontrol achieved in reaction to the two command signals 77 and 100results in torque distribution as illustrated in FIG. 2B when the wheelslip of the front wheels is greater than wheel slip of the rear wheels.In addition, the control technique results in better integration of thecontrol provided by the front and rear axle controllers 78 and 76, inwhat can be termed a “cooperative” approach.

FIG. 4 illustrates an embodiment of cooperative traction controlimplemented in a traction control module designed for use in a vehiclethat is primarily rear-wheel drive. As can be seen, in this embodimenttorque is transferred from the rear wheels to the front wheels in amanner that is similar to the situation described above with respect toFIGS. 2A, 2B, and 3. Since there are similarities between the primarilyfront-wheel and primarily rear-wheel drive modalities, no furtherdiscussion of FIG. 4 is provided.

Thus, the invention provides, among other things, a traction controlmodule in which the transfer of torque from, for example, the frontwheels to rear wheels, is controlled by two controllers each of whichperforms control on an axle-by-axle basis (i.e., control to both wheelsconnected to a front axle and control to both wheels connected to a rearaxle). Various features and advantages of the invention are set forth inthe following claims.

1. A traction control module for a vehicle having a first axle, a secondaxle, and an engine that produces torque, the traction control modulecomprising: a first comparator that receives a first-axle, left wheelslip value and a first-axle, right wheel slip value, the firstcomparator generating a first axle slip value indicative of a greater ofthe first-axle, left wheel slip value and the first-axle, right wheelslip value; a second comparator that receives a second-axle, left wheelslip value and a second-axle, right wheel slip value, the secondcomparator generating a second axle slip value indicative of a greaterof the second-axle, left wheel slip value and the second-axle, rightwheel slip value; a first summing node that receives the first axle slipvalue and a first axle target slip value and that generates a first sliperror; a second summing node that receives the second axle slip valueand a second axle target slip value and that generates a second sliperror; a first axle controller that generates a first axle torquecommand output based on the first slip error; and a second axlecontroller that generates a second axle torque command output based onthe second slip error.
 2. The traction control module of claim 1,further comprising a third comparator that receives the first axletorque command output and an actual first axle torque value andgenerates an output indicative of the lesser of the first axle torquecommand output and the actual first axle torque value.
 3. The tractioncontrol module of claim 2, further comprising a third summing node thatreceives the output of the third comparator and the second axle torquecommand output and generates a difference output.
 4. The tractioncontrol module of claim 3, further comprising: an engine controller thatproduces an engine target torque value; and a fourth summing node thatreceives the difference output and the engine target torque value andgenerates an engine torque command.
 5. The traction control module ofclaim 4, further comprising a controllable torque distribution device,and wherein the engine torque command is provided to the enginecontroller to control an engine torque output and the second axle torquecommand is provided to the controllable torque distribution device. 6.The traction control module of claim 5, wherein an effect of the enginetorque command and the second axle torque command is to reduce anoverall torque produced by the engine and distribute more torque to thesecond axle than to the first axle in a situation where the first axleslip value is greater than the second axle slip value.
 7. A method ofproviding fraction control in a vehicle having a first axle, a secondaxle, and an engine that produces torque, the method comprisingdetermining, with a traction control module, a left first-axle wheelslip value, a right first axle wheel slip value, a left second-axlewheel slip value, and a right second-axle wheel slip value; determining,with a first comparator, a first axle slip value by comparing the leftfirst-axle wheel slip value and the right first-axle wheel slip value;determining, with a second comparator, a second axle slip value bycomparing the left second-axle wheel slip value and the rightsecond-axle wheel slip value; determining, with a first summing node, afirst slip error based on the first axle slip value and a first axletarget slip value; determining, with a second summing node, a secondslip error based on the second axle slip value and a second axle targetslip value; generating, with a first axle controller, a first axletorque command output based on the first slip error; and generating,with a first axle controller, a second axle torque command output basedon the second slip error.
 8. The method of claim 7, further comprising:comparing, with a third comparator, the first axle torque command outputand an actual first axle torque value and generating, with the thirdcomparator, an excess torque output that is indicative of a lesser ofthe first axle torque command output and the actual first axle torquevalue; determining, with a third summing node, a difference output basedon a difference between the excess torque output and the second axletorque command.
 9. The method of claim 8, further comprising: generatingan engine target torque value with an engine controller; anddetermining, with a fourth summing node, an engine torque command basedon the difference output and the engine target torque value.
 10. Themethod of claim 9, further comprising: providing the engine torquecommand to an engine controller to control a torque output of theengine; and providing the second axle torque command output from thesecond axle controller to a controllable torque distribution device inthe vehicle.
 11. A traction control module for a vehicle having a frontaxle, a rear axle, and an engine that produces torque, the tractioncontrol module comprising: a first comparator that receives a left frontwheel slip value and a right front wheel slip value, the firstcomparator generating a front axle slip value indicative of a greater ofthe left front wheel slip value and the right front wheel slip value; asecond comparator that receives a left rear wheel slip value and a rightrear wheel slip value, the second comparator generating a rear axle slipvalue indicative of a greater of the left rear wheel slip value and theright rear wheel slip value; a first summing node that receives thefront axle slip value and a front axle target slip value and thatgenerates a first slip error; a second summing node that receives therear axle slip value and a rear axle target slip value and thatgenerates a second slip error; a front axle controller that generates afirst axle torque command output based on the first slip error; and arear axle controller that generates a second axle torque command outputbased on the second slip error.
 12. The traction control module of claim11, further comprising a third comparator that receives the first axletorque command output and an actual front axle torque value andgenerates an output indicative of a lesser of the torque first axletorque command output and the actual front axle torque value.
 13. Thetraction control module of claim 12, further comprising a a thirdsumming node that receives the output of the third comparator and thesecond axle torque command output and generates a difference output. 14.The traction control module of claim 13, further comprising: an enginecontroller that produces an engine target torque value; and a fourthsumming node that receives the difference output and the engine targettorque value and generates an engine torque command.
 15. The tractioncontrol module of claim 14, further comprising a controllable torquedistribution device, and wherein the engine torque command is providedto the engine controller to control a torque output of the engine andthe second axle torque command output is provided to the controllabletorque distribution device.
 16. The traction control module of claim 15,wherein the effect of the engine torque command and the second axletorque command output is to reduce an overall torque produced by theengine and distribute more torque to the rear axle than to the frontaxle in a situation where the front axle slip value is greater than therear axle slip value.
 17. A method of providing fraction control in avehicle having a front axle, a rear axle, and an engine that producestorque, the method comprising: determining, with a fraction controlmodule, a left front wheel slip value and a right front wheel slipvalue; comparing, with a first comparator, the left front wheel slipvalue and the right front wheel slip value and generating a front axleslip value that is indicative of a greater of the left front wheel slipvalue and the right front wheel slip value; determining, with thetraction control module, a left rear wheel slip value and a right rearwheel slip value; comparing, with a second comparator, the left rearwheel slip value and the right rear wheel slip value and generating arear axle slip value that is indicative of the greater of the left rearwheel slip value and the right rear wheel slip value; determining, witha first summing node, a first slip error based on the front axle slipvalue and a target slip value for the front axle; determining, with asecond summing node, a second slip error based on the rear axle slipvalue and a target slip value for the rear axle; generating, with afront axle controller, a first torque command output based on the firstslip error; and generating, with a rear axle controller, a second torquecommand output based on the second slip error.
 18. The method of claim17, further comprising: comparing, with a third comparator, the firsttorque command output and an actual front axle torque value andgenerating, with the third comparator, an excess torque output that isindicative of the lesser of the first torque command output and theactual front axle torque value.
 19. The method of claim 18, furthercomprising: determining, with a third summing node, a difference outputbased on the difference between the excess torque output and the secondtorque command output.
 20. The method of claim 19, further comprising:generating an engine target torque value with an engine controller; anddetermining, with a fourth summing node, an engine torque command basedon the difference output and the engine target torque value.
 21. Themethod of claim 20, further comprising: providing the engine torquecommand to the engine controller to control a torque output of theengine; and providing the second torque command output to a controllabletorque distribution device in the vehicle.